6002
Study of Ion-Molecule Reactions and Reaction Mechanisms in Acetonitrile by Ion Cyclotron Resonance' George A. Gray2
Contribution from the Department of Chemistrj),Stanford Unicersity, Stanford, California 94305. Received April 29, 1968 Abstract: The ion-molecule chemistry of acetonitrile has been investigated using ion cyclotron resonance. Twenty-three ion-molecule reactions are observed in acetonitrile and nineteen others involving acetonitrile in gaseous mixtures utilizing pulsed double resonance experiments. For several of these reactions, direct information is obtained regarding the detailed mechanisms by using isotopically enriched materials. Evidence is presented for the reaction of vibrationally excited methyl ion originating from electron impact on acetonitrile.
T
he contradictions and ambivalence evident in prior investigations"-6 suggest that the ion-molecule chemistry of acetonitrile has by no means been completely delineated. Furthermore, acetonitrile has been found in discharge chambers containing reducing atmospheres, suggesting that an understanding of the ionmolecule chemistry of this system might be relevant to an understanding of abiogenic s y n t h e ~ i s . ~ -Re~ cently, a number of studies have been conducted using ion cyclotron resonance (icr)10-13 which have been extremely successful in the analysis and interpretation of complex ion-molecule reaction chemistry. The principal advantage of this approach is the high ionmolecule reaction probability resulting from the very long ion path lengths attainable in this spectrometer. Multiresonance techniques greatly simplify the task of sorting out complex ion-molecule chemistry. Ionmolecule reactions are observed directly, thus removing the cause of error in normal mass spectrometric investigations. Consider a reaction A+ B + Cy- D. The signal from C+ can be continuously monitored while an irradiating radiofrequency field is swept through a range of cyclotron frequencies. When the irradiating frequency matches the natural cyclotron frequency of A+, energy is absorbed by A+ from the irradiating field and the translational energy of A+ is increased. Since most reaction rate constants are dependent on the ion velocity, the number density of C+ will change. If the irradiating field is pulsed on and OK and the cyclotron resonance of C+ is detected by a phase-sensitive detector referenced to the pulsing frequency, this change in
+
+
(1) This work was supported by the National Science Foundation under Grant G P 4924-x, the National Aeronautics and Space Administration under Grant NGR-05-020250, and the Center for Materials Research, Stanford University. (2) National Institutes of Health Postdoctoral Fellow, 1967-1968. Address correspondence to the author at the Oregon Graduate Center, Portland, Oregon 97225. (3) T , W. Martin and C. E. Mclton, J . Chern. Ph.rs., 32, 700 (1960). (4) T. F . Moran and W. H. Hnmill, ibid.,39, 1413 (1963). (5) T. W. Shannon and A. G . Harrison, ibid., 43, 4206 (1965). (6) J. L. Franklin, Y . Wada, P. Natalis, and P. M . Hierl, J . Phys. Chem., 70, 2353 (1966). (7) S . L. Miller, J . Amer. Chem. Soc., 77, 235 (1955). (8) S . L. Miller and H. C. Urey, Science, 130, 245 (1959). (9) L. Francesconi and A . Cirulo, Garr. Chim. Ital., 53, 327 (1923). (10) L. R . Anders, J. L . Beauchamp, R. C. Dunbar, and J. D. Baldeschwielcr, J . Chem. Phj.s., 45, 1062 (1966). (11) .I.L. Beauchamp, L. R . Anders, and J. I). Baldeschwieler, J . Anier. Chem. Soc., 89, 4569 (1967). (12) J. D. Baldeschwisler, Science, 159, 263 (1968). (13) J . L. Beauchamp and S . E. Buttrill, Jr., J . Chem. Phys., 48, 1783 (1968). (14) J. M . Hcnis, J . Anter. Chem. SOC.,90, 844 (1968). (15) G . A. Gray, ibid.,90, 2177 (1968).
Journal of the American Chemical Society
ion number density can be directly presented. If ion C+ is observed at its cyclotron frequency while the irradiating frequency w2 is varied, signals will be observed for values of w 2 corresponding to the cyclotron resonance frequencies of reactant ions which yield the product C+. This experiment can be repeated for any combination of ions, providing a map of the entire chemistry of a mixture of ions and neutral molecules. In this paper ion cyclotron resonance is used to sort out the ion--molecule reactions occurring in gaseous acetonitrile following electron-impact generation of molecular ions. The inconsistencies in the previous descriptions of the self-protonation reaction are resolved. Certain of the previously reported reactions are confirmed while many new ion-molecule reactions are revealed. In addition, icr experiments with labeled acetonitrile yield intriguing implications regarding reaction mechanisms and ion structures. Experimental Section Single and Double Resonance. The icr spectra were obtained using a Varian V-5900 icr spectrometer. The basic spectrometer and the application of single and double resonance have been described p r e v i o ~ s l y . ~ ~The - ~ ~phase convention used in this paper for the pulsed double resonance signals is that positive signals correspond to increases in the observed ion number density at the condition of double resonance, while the opposite holds true for negatively directed double resonance signals. The collection of meaningful double resonance data requires caution in the selection of two instrumental parameters. These are the electron beam current and the intensity of the double irradiation field. At high current and/or high double irradiation fields misleading double resonance signals can occur. High emission currents can lead to poorly understood space charge effects which can result in large positive double resonance signals, even when the reaction under observation shows a negative double resonance signal at low emission currents. To avoid misassignments the lowest practical emission current should be used, usually 0.2 PA or lower. When double resonance field intensities become much greater than approximately 0.04 V/cm there is a possibility of obtaining the incorrect sign for the double resonance. For example, consider a case where a double resonance signal is weakly positive for the reaction 'x -+ y + and strongly negative for the reaction (x i I)+ y-. The double resonance for x+-+ y+ can be influenced by the (x i 1)y + double resonance signal because of the width of the output band of the doub1e:resonance oscillator, such that at high intensities of double resonance irradiation, the overlapping negative contriy+ outweighs the positive contribution bution from (x i I)+ from x+. For this reason the lowest double resonance field intensities should be employed and varied (as in Figure 4) to check for sign reversal. Materials. Matheson (Spectro Grade) acetonitrile was used without further purification, as was the 99.5 perdeuterioacetonitrile, purchased from Stohler Isotope Chemicals Co., Azusa, Calif. 91702. Samples were normally prepared as the neat liquids or --f
-f
1 90:22 1 October 23, 1968
-f
6003 mixtures of liquids in air. The air was removed from the sample by using the roughing pump of the spectrometer. This procedure was repeated not less than five times for each sample to ensure complete removal of air. Even with these precautions a residual amount of nitrogen gas remained and contributed to the m/e 28 resonance and in its protonated form to the m/e 29 resonance. Traces of argon at m/e 40 and protonated argon at m/e 41 were obscured by the acetonitrile resonances. Studies using deuterated acetonitrile confirmed the presence of nitrogen. The acetonitrile-l5N was prepared from KCl5N (Isomet) and CHJ (Matheson Spectro Grade) using an adaptation16 of the procedure of Brand~trom.~'The sample consisted of a 4 : l mixture of CH3C14N-CH3C16N, with no other significant impurities, as shown by proton nmr and icr.
CN-
I
C H,C
N
Results and Discussion CH,CN + H,O Positive Ion Spectra. Table I compares the primary CNions of the icr spectrum with the data of Franklin, et a1.6 The only significant differences are the higher relative intensities of the mje 14, 26, and 27 peaks in the C H,CNmass spectrometric work. Table I1 lists the secondary ions observed. Assignments were made on a formula weight basis but structural suggestions for a few of the ions are also given in Table 11. The structure of mje 42 is consistent with what would be expected from a chemical viewpoint, although alternative structures are possible. Structures for mje 50, 54, 81, and 83 are assigned on the basis of the double resonance data, to be discussed below. Structures for mje 64-66 are those proposed by Franklin, et al.,'j while that for mje Figure 1. A, Single resonance negative ion spectrum of acetonitrile 67 seems chemically reasonable. Franklin, et ~ l . , ~at 10-5 torr using 3-eV ionizing voltage; B, single resonance negative assigned m/e 83 as the proton-bound dimer, reflecting ion spectrum of an acetonitrile-water mixture at 1 0 - 6 torr using 8-eV ionizing voltage. a general behavior of organic bases. Table I. Positive Ions Produced in Acetonitrile ( 6M
-
mle
This work
Assignment
+ 1 +)
Relative intensity
-
Ref 65
___
~~
12 13 14 15 16 17 18 24 25 26 27 28 29 38 39 40 41 42
C.+ CH+ CH2 and/or N+ CH3+and/or NH CHI. and/or NH2+ NH3.' NHa+ +
1
+
c2.+
CzH+ C2H2. and/or CN+ CZH3+ and/or H C N . + HzCN+and/or N z . + NzH+ CCN+ CHCN.+ CHzCN+ CHZCN ' CH&NH+ +
+
0.7 0.5 3.2 3.8 0.5 0.5 0.2 0.4 1.0 0.4 5.6 10.8 1.1 13.2 22.7 85.7 100 49.4
peak observed. No molecular ion peaks or other fragment ions are observed in pure CH3CN. The other intense resonance is that of m/e 50. This resonance is
3.5 2.4 12.5 6.9
Table 11. Secondary Positive Ions Produced in Acetonitrile ( 2M 1 +)
+
m/e
42
Structure assignment CHaCNH+ CCNC+
IS.@
51 52 53 54
CH*CNCHz+
8.1
55
9.6 18.1 79.0 100 73.9
At 30 k. Approximately 60% CN+, 40% CZHZ.". Approximately 85 % HCN .+,15% ,CzH3+. a
Negative Ion Spectra. Negative ion spectra were run employing very low electron energies, usually on the order of 3 eV, with electron currents around 2 PA. Low drift potentials (e.g., 0.02 V/cm) were used and spectra like that in Figure 1A were attained. Table I11 lists the observed ions. Cyanide ion is the major (16) G . A . Gray, Thesis, University of California, Davis, Calif., 1967, (17) A. Brandstrom, Acta. Chem. Scand., 10, 1197 (1956). (18) J . L. Beauchamp, Thesis, Harvard University, 1968.
Relative intensity
50
0.4 2.6 8 . I*
p 2s.
Composition
56 64 65 66 67 68 79 80 81 82 83 84 91 93 95 96
C(CN)z CH(CNz)+ CHz(CN)1. NCCHzCNH+ +
+
NCCHNCCHz+ NCCHzNCCHat H(CH3CN)zf
unaffected wken mixtures of CHsCN and CD,CN are studied, indicating that the species contains no hydrogen atoms. A decrease in the number density of mle 50 Gray / Ion Cyclotron Resonance
6004 Table 111. Negative Ions Produced in Acetonitrile m/e
Composition
26 50 65 74 79
CNC3NC~HNPCjNCiiH3Nz-
mje 50 is the second most prominent ion in the negative ion spectrum. Pulsed double resonance also shows that the ion m/e 65 is also formed from cyanide ion. Reactions. Establishing a set of reactions was, in some cases, uncomplicated and straightforward. However, certain reactions had parent ion masses that could be assigned to more than one species. Reactions 14-18 of Table IV were of this type. The following experi-
Structure assignment CENC=C-C=NHC(CN)zC=C-*C-C=NNEC-CH-CH-C~N-
Table IV. Reactions Observed Acetonitrile
+
-
AHrxZ08 ', kcal/mol
(dkldEion)"
+
+ + + + + +
60 60
C5HGN+ CHSCN C3HaN+ CiHz CHICN CH3CNH+ CH3CN (CHsCN)zH+ HCN CH3CNHT CH3CN + CsHaN+ Hz CHaCNH+ CHICN + CH3CNH+ CH3CN C3HaN+ HCN H. CH3CN.+ CHaCN CHaCN -L CH3CNH+ CHzCN. CH3CN.t CHzCN+ CH3CN + CaHsNz+ CHzCN+ 2 CHaCN + CiHsN+ CHzNz CaHaN+ HCN CHZCN+ CHICN CHZCN+ CHaCN --L CHaCNH+ CHCN CHCN.+ CHaCN + CaH3Nz+ H . CHCN.+ CH3CN -+ CaH$N.+ HCN CHSCN -* CH3CNH+ CCN CHCN . HPCN+ CH3CN -+ CH3CNH+ HCN CH3CNH+ CN. HCN. + + CH3CN HCN.+ + CH3CN -+ CHsCN.+ HCN CzH. CZHz.+ CHICN CH3CNH' CH3CN CHaCN'+ CzHz CZH2.f CH3CNHC CHz CH3*+ + CH3CN C3HaN+ H . CHz.+ CH3CN CHZ.+ CHSCN -t CH3CNH+ CH. CH3CN -+ C3NNH3 CNC3HNzHP CN- + CH8CN -+
+ + + + + + + + + + + + +
-+
-+
-+
-+
-+
-+
+ CHICN + CHICN + CH3CN
OHOH-
+ CH3CN
+ CHaCN
60
-73 - 14 - 63 - 38 - 23 +19
60
- 30 -11
60 $0
+ N1
+ + + + Acetonitrile + HzO NCO- or CNO- + CHI CHzCN- + HzO Acetonitrile + CH31
- 73 - 47 - 77
CH3CNH+ NZ CHzCN' NZ H . CHaCN'+ NZ
--L
-+ -+
+ 1.
-+
-+
Acetonitrile CH3CNH+ CN. CH3CNH+ HCN H,CN+ CHaCN
+
-+
+ +
Acetonitrile CHZOH+ CH3CN CH,CNH+ [HzCO] CHaOH.+ CH3CN + CHaCNH+ CHIO' CHICN + CH3CNHCHaOH CH30Hz' CH3OHz' CHaCN CH3CNH+ + CHIOH
+ + +
60
-+
-+
HCN.+ CH3CN HZCN+ CH3CN CH3CNH+ HCN
+
+5
Acetonitrile
CHaCN.+ + CHI1 + CHaCNCH,+
+ +
60 60 0
+ + + + + + + + +
-+
NZH+ N2,+ NZ.'
+
30 - 69
+ + + + + + + +
-+
+
+ + + + +
+81 0
+ + + +
-+
--L
+ +
CH3CHOHC CHICN CHaCNH+ CHICHO
